Extracorporeal support circuits are used to provide circulatory support to patients during cardiovascular surgery. A support circuit of this type includes a venous line for draining blood from the right side of the patient's heart. The venous line delivers the blood to a blood reservoir, such as the venous reservoir sold under the trade designation "SARNS" (Catalog No. 9445) by the Minnesota Mining and Manufacturing Company, St. Paul, Minn. Blood is typically pumped, via a specially designed blood pump, from the outlet of the blood reservoir into a blood oxygenator, such as the oxygenator and heat exchanger sold under the trade designation "SARNS MEMBRANE OXYGENATOR" or "SMO" heat exchanger (Part No. 16310) by the Minnesota Mining and Manufacturing Company, St. Paul, Minn., for oxygenation and cooling. The oxygenated blood is then delivered via an arterial line to the patient. While the pump is running, the patient returns blood to the venous line to repeat the cycle.
The support circuit normally also includes a blood scavenging sub-circuit for recovering blood from the surgical field to recycle the blood. The scavenging sub-circuit includes one or more suckers (typically two to four) for sucking blood from the surgical field. Vacuum is applied to the suckers by a peristaltic positive displacement pump (also known as a roller pump) or wall vacuum to deliver the scavenged blood to a cardiotomy reservoir. A cardiotomy reservoir includes a defoaming section because the scavenged blood normally includes a large amount of entrained air, and a filter for filtering the scavenged blood. The outlet for the cardiotomy reservoir delivers the de-foamed, filtered blood to the venous blood reservoir of the main circuit. U.S. Pat. Nos. 3,891,416; 3,993,461; 4,208,193; and 4,243,531 show various cardiotomy reservoirs.
The cardiotomy reservoir may alternatively be an integral portion of the venous blood reservoir, in which the scavenged blood flows through a filter section and the venous blood does not. Both the scavenged blood and venous blood would flow through a defoaming section. Examples of such combined venous and cardiotomy reservoirs are sold under the following trade designations: "SARNS MEMBRANE OXYGENATOR WITH INTEGRAL CARDIOTOMY RESERVOIR" or "SMO/ICR" (Catalog No. 9462) and "SARNS FILTERED VENOUS RESERVOIR" (Catalog No. 9438) by the Minnesota Mining and Manufacturing Company, St. Paul, Minn.; "HSVRF" hardshell venous reservoir with integral cardiotomy filter by Shiley Inc., Irvine, Calif.; and "BARD WILLIAM HARVEY H-4700 SERIES" cardiotomy reservoir by C. R. Bard, Inc., Billerica, Mass.
Of course, there are numerous permutations of the basic circuit and sub-circuit, in addition to those described above, that have been employed to provide circulatory support.
In extracorporeal support systems of the type described, there has long been interest among other things in (a) minimizing the prime volume of the circuit, (b) minimizing damage (hemolysis) of the blood circulated through the circuit, (c) preventing air emboli being formed in the circuit and delivered to the patient, and (d) preventing loss of prime in the circuit. As a general rule, the lower the prime volume of the circuit, the less donated blood is required. As a result, reducing the prime volume also reduces demand on donated blood supplies, as well as reducing various risks associated with diseases, such as aids and hepatitis B, that can be transmitted by blood notwithstanding careful testing of the blood supply. Reducing hemolysis and preventing air emboli are also widely recognized as beneficial or necessary.
In specifying the minimum fluid volume in venous blood reservoirs, the blood level relative to the outlet of the reservoir has typically been the major consideration. The minimum blood level in such reservoirs has been set high enough relative to the outlet to prevent a vortex being formed by blood exiting the reservoir. A vortex would be undesirable in that with a vortex air might be entrained in the blood exiting the reservoir.
In many venous blood reservoirs, the venous blood inlet is positioned above the minimum blood level, and a defoaming section is provided in the flow path between the inlet and outlet of the reservoir. One reason for the arrangement of the inlet above the minimum blood level is to reduce the time and amount of blood that is in contact with the synthetic material of the defoaming section of the reservoir. When the pump of the main circuit is running, air migration up the inlet is not believed to be a problem. In order to prevent air from migrating up the inlet when the pump is off and to prevent loss of prime in the venous line, it has been the perfusionist's or surgeon's responsibility to clamp the venous line with a suitable clamp (e.g., hemostat) when the pump is turned off to close the venous line to any flow. If both the perfusionist and surgeon fail to clamp the venous line when the pump is turned off, there is a chance that the venous line will lose its prime, with the result that the perfusionist may need to re-prime the venous line before the pump could be restarted.
An integral blood reservoir, heat exchanger and blood oxygenator is available under the trade designation "CML" (Catalog No. 050-104-000) from Cobe Laboratories, Inc., Lakewood, Colo. See, also, U.S. Pat. Nos. 4,818,490 and 4,656,004. That reservoir includes an inlet drop tube extending downwardly into a defoaming element within the reservoir. The lower, downstream end of that drop tube is surrounded in the horizontal direction by an annular flange extending upwardly from a lower surface of the supporting structure for the defoaming element. The annular flange of the "CML" brand reservoir may form a pool of fluid adjacent the downstream end of the drop tube with some fluid trapping action that might help maintain prime in the venous line.